Transparency mode (a.k.a Hearing Through, Aware Mode, Ambient Mode, they are all the same feature that lets in or even amplify the outside sound) is integrated as TWS earbuds' feature that let wearers hear the ambient sound while they listen to music. Because earbuds physically block out the noises and wearers sometimes miss out the important sound they would need to hear.
Apple was not the first one introducing this feature to the market, but they did implement it in their AirPods and Airpods Pro lineups brilliantly thanks to their in-house chip H1 and H2. Listening ambient sound through AirPods Pro 2 feels natural, clear and transparent and its quite close as if you weren't wearing earbuds. But Android users won't have or access some spacialized functions and battery status.
Other brands are about to to catch up Apple in term of transparency feature though they don't have custom SoC, and they have to reply on the SoC manufacturers like Qualcomm, MediaTek and Broadcom which already developed a comparable technology for voice processing. But how they are implemented in earbuds will determine the performance, that's why we see a lot of variation in performance between models.
To improve and optimize the voice processing for transparency modes, brands like Sony worked with MediaTek to customize the SoC for better algorithms but it's uncommon.
Note: A lot of people recommend Sony LinkBuds S, but acctually they are just open and not comparable to sealed earbuds in term of design.
